The invention relates to threaded joints for well casing and tubing and, more particularly, to a threaded joint in which joint strength, sealing capabilities, ease of assembly, and resistance to stress corrosion, among other things, are enhanced over prior threaded joints.
Oil and natural gas wells currently are being driven to depths in excess of 5,000 feet. In the drilling of such wells, sections of tubing connected by threaded couplings and having a drill bit at the lower end progressively are advanced into the earth. After the well has been drilled to its final desired depth, or at some point during the course of the drilling operation, a casing having a diameter greater than the diameter of the tubing is driven into the earth. The casing surrounds the tubing and prevents the sidewalls of the hole from collapsing. Generally the casing does not extend very far into the earth, usually only far enough to ensure that the sidewalls of the hole are sufficiently strong that the walls will not collapse. In the description to follow, the term "pipe section" will be used to refer generally to tubing or casing sections, or both, as the case may be.
It will be understood that in a string of tubing, which may extend to a depth approaching 5,000 feet, the uppermost tube joint must withstand the total weight of the tubing, even though no external forces are exerted and the tubing simply is suspended. Since the ordinary tubing used today weighs from 8 to 60 pounds per foot of tubing length, a tremendous load must be carried by the joint threads. If the tubing also is used for the purpose of drilling the well (as opposed to merely conducting fluid from the well), the joint must be able to withstand high torque loads in addition to the tensile load.
Another consideration is that a well rarely is driven exactly in a straight line from the surface of the earth to the bottom of the well. Obstructions almost certainly will be encountered during the drilling operation, and slight deviations from the intended drilling path will occur. The tubing used to drill the well and/or convey fluid from the well must be able to follow the course of the well as greater depths are reached. This places bending loads on the tubing at various places along its length and produces higher stress than would be encountered if the well were drilled in exactly a straight line.
Because the well deviates from a straight line and because the tubing may be urged downwardly during the drilling operation, the tubing actually may be under compression for periods of time. This is particularly so with casing sections because they often are driven into the well. Although the threaded joints must be able to withstand tremendous tensile loads, the joints must be able to absorb considerable compression loads also. Threaded joints have been known to "ratchet" under extreme compression; that is, the threads may be forced past each other without the pipe sections being rotated relative to the coupling. Ratcheting can lead to catastrophic failure of the joint, and even if complete failure does not occur, the threads may be deformed sufficiently that fluid leakage is likely.
Other considerations must be taken into account in designing a threaded joint having acceptable characteristics. For example, it is important that the joint provide a seal against leakage between mating threaded elements. This may be achieved by providing, upon makeup of the joint, a metal-to-metal seal between interfitting or interengaging faces of male and female threaded elements of the joint. It is important that the mating portions be generally free of defects or blemishes because, unless substantial surface-to-surface contact is maintained, leakage likely will occur. This is particularly so in the case of a very deep well due to the extreme fluid pressures involved and because the considerable jostling which the tubing undergoes tends to loosen the threaded connections and thereby lead to a leakage condition.
Problems occur in assembling the joints in that the components to be assembled are so heavy and unwieldy that it is difficult to handle them without damage, particularly as a new pipe section is being "stabbed" into a coupling. The result of this is that the threads easily can be blemished or the joints can be threaded improperly, which if carried far enough, can result in destruction of the threaded connection and require that new coupling and pipe sections be substituted. In short, it not only is desirable that the components be capable of being assembled quickly, but also that they can sustain damage from handling and stabbing.
A part of the difficulty in this area arises from the techniques employed to assemble the joint components. Because the pipe sections are so heavy and unwieldy, machinery must be employed to assist in assembling the components at the job site. This machinery not only lifts the new section to be added to the already-connected sections, but the machinery also rotates the new section to secure the threaded connection. A common difficulty has been that of accurately controlling the torque applied to the pipe section so that portions of the joint are not overstressed. Accordingly, it is desirable that a positive stop be provided so that when a predetermined level of thread engagement has been reached, rotation of the pipe section will be stopped abruptly. Although this capability has existed for some time, the various prior threaded joints have not been designed to account for different stress levels reached during the tightening process. That is, the coupling process produces hoop stresses in the connected components and it has been found that existing joint designs produce higher hoop stresses in smaller diameters with equal power-tightening travel applied to a range of sizes. This obviously increases the risk of premature failure unless service ratings are reduced along with size. One approach to this problem has been to make the smaller-diameter couplings proportionately bulkier simply to withstand the higher anticipated hoop stresses.
Reduction of hoop stress is important, not only to optimize the service pressure rating of the joint and to reduce the size of the joint components, but also to reduce stress corrosion in acidic wells. Although the patentee does not wish to be bound by a particular theory, it is generally known that tubing components corrode faster with increasing stress levels when maintained in contact with acidic compositions.